The present invention relates to a secondary battery including a spirally wound body formed through laminating a cathode and an anode with an electrolyte in between, and spirally winding the cathode and the anode, and more specifically to a secondary battery including an anode which includes an anode active material being capable of inserting and extracting an electrode reactant and including at least one kind selected from the group consisting of metal elements and metalloid elements as an element.
In recent years, a large number of portable electronic devices such as camcorders, cellular phones and laptop computers have been emerged, and an attempt to reduce the size and the weight of them has been made. Research and development aimed at improving the energy densities of batteries used as portable power sources of the electronic devices, specifically secondary batteries as a key device have been actively promoted. Among the batteries, a nonaqueous electrolyte secondary battery (for example, a lithium-ion secondary battery) can obtain a high energy density, compared to a lead-acid battery and a nickel cadmium battery which are aqueous electrolyte secondary batteries in related arts, so the improvement of the nonaqueous electrolyte secondary battery has been studied in all quarters.
As an anode active material used in the lithium-ion secondary battery, a carbon material having a relatively high capacity and superior cycle characteristics such as non-graphitizable carbon or graphite is broadly used. However, in consideration of a recent demand for a higher capacity, a further increase in the capacity of the carbon material presents a challenge.
In such a background, a technique of achieving a higher capacity by the use of a carbon material through selecting a material to be carbonized and forming conditions has been developed (for example, refer to Japanese Unexamined Patent Application Publication No. H8-315825). However, when such a carbon material is used, an anode has a discharge potential of 0.8 V to 1.0 V relative to lithium, and when a battery includes the carbon material, the discharge voltage of the battery is reduced, so a significant improvement in the energy density of the battery can be hardly expected. Moreover, there is a disadvantage that hysteresis in the shape of a charge-discharge curve is large, thereby energy efficiency in each charge-discharge cycle is low.
On the other hand, as an anode with a higher capacity than the carbon material, an alloy material which is formed through electrochemically alloying some kind of metal with lithium and has a property of being reversibly produced and decomposed has been researched. For example, an anode with a high capacity using a Li—Al alloy has been developed, and an anode with a high capacity including a Si alloy has been developed (for example, refer to U.S. Pat. No. 4,950,566). Moreover, an intermetallic compound Cu6Sn5 has been developed (for example, refer to D. Larcher “Journal of The Electrochemical Society” 2000, Vol. 5, No. 147, p. 1658-1662).
However, a Li—Al alloy, a Si alloy or Cu6Sn5 has a big disadvantage that the cycle characteristics are extremely poor, because the alloy expands or shrinks according to charge and discharge, so every time a charge-discharge cycle is repeated, the anode is pulverized.
Therefore, as a technique for improving the cycle characteristics, a technique of substituting an element not involved in expansion and shrinkage according to insertion and extraction of lithium for a part of the alloy has been considered. For example, LiSisOt (0≦s, 0<t<2), LiuSi1-vMvOw (where M is a metal element except for alkali metal or a metalloid element except for silicon; 0≦u; 0<v<1; and 0<w<2), or a LiAgTe-based alloy has been proposed (for example, refer to Japanese Unexamined Patent Application Publication Nos. H6-325765, H7-230800 and H7-288130). However, the fact is that even if these anode active materials are used, a decline in charge-discharge cycle characteristics due to expansion and shrinkage is large, so it is difficult to take advantage of a high capacity.
Moreover, in the case where these anode active materials are used, an anode largely expands or shrinks according to charge and discharge. Therefore, specifically in the case where a spirally wound laminate including a cathode and an anode with an electrolyte in between is used, a step formed by an end portion of an active material layer on an outer side of the spirally wound laminate or a lead presses the separator by the expansion of the anode, thereby the anode comes into contact with the other electrode to develop a very small short circuit, and the short circuit causes a decline in charge-discharge cycle characteristics. Moreover, abrasion of an end portion of an active material layer and paint splatters during formation cause a step on the surface of the cathode or the anode, thereby a very small short circuit may occur in a like manner to cause a decline in the charge-discharge cycle characteristics. Such a phenomenon occurs specifically under high temperature conditions or during a charge-discharge cycle in a state of overcharge.
In a winding type battery using a carbon material as an anode active material in a related art, a technique of adhering an insulating material such as a tape made of polyimide or polypropylene to an end portion of a first electrode and a second electrode facing the end portion of the first electrode in order to prevent a short circuit between the electrodes which may occur in the case where an external pressure is applied to the battery has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-266946).
Another factor causing a decline in the charge-discharge cycle characteristics is low roundness of a spirally wound body. When the roundness of the spirally wound body is low, and the distortion of the spirally wound body is large, a pressure is not uniformly applied to an electrode. The separator in a position where a high pressure is applied is pressed, thereby a very small short circuit may occur. On the other hand, in a position where a low pressure is applied, a gap between electrodes expands, thereby the deposition of lithium may occur to cause a serious decline in load characteristics or the charge-discharge cycle characteristics. Further, in the case where the spirally wound body is put into a relatively soft battery can, the battery may be deformed by an pressure from inside caused by the expansion of the anode.
In a related art, in order to improve the roundness, a technique of arranging a cathode lead and an anode lead such that the cathode lead and the anode lead form a central angle of approximately 120° or 240° at the center of the spirally wound body has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-87324); however, a further improvement in the roundness is desired.
Moreover, there is a disadvantage that the deterioration of an electrode occurs during a charge-discharge cycle by repeats of the expansion and shrinkage of an anode active material, and accordingly the electrode may be broken during the use of the battery to cause an interruption of use or a short circuit. In particular, in the case where a spirally wound laminate including a cathode and an anode with an electrolyte in between is used, when steps formed by an end portion of an active material layer or a lead gather in one point because of a structural reason, that is, a small diameter of the spirally wound laminate on a center side, the shape of the spirally wound laminate is largely distorted, thereby the possibility that the electrode is broken due to the deterioration of the electrode is increased. This issue is serious specifically in a high load charge-discharge cycle or a charge-discharge cycle in a state of overcharge.
In a winding type battery using a carbon material as an anode active material proposed in a related art, the cathode is disposed at an outermost position of a spirally wound body, and an outermost end portion of an anode current collector is protruded from an outermost end portion of a cathode current collector toward the front, and an anode lead is attached to the protruded portion of the anode current collector, thereby the unevenness of the outermost portion of the spirally wound body can be eliminated, and a short circuit between the anode lead and the outermost end portion of the cathode current collector can be prevented (for example, refer to Japanese Unexamined Patent Application Publication No. H11-260415).